1. Field of the Invention
The present invention relates to an electrophotographic photoconductor mounted in, for example, electrophotographic printers and copiers, and specifically to an electrophotographic photoconductor which is excellent in response, which is applicable to high-speed processing apparatus and which has high ozone and NOx resistances.
The present invention also relates to an electrophotographic method using the electrophotographic photoconductor, an electrophotographic apparatus containing the electrophotographic photoconductor and a process cartridge for use in an electrophotographic apparatus containing the electrophotographic photoconductor.
2. Description of the Related Art
Electrophotographic photoconductors (hereinafter may be referred to as “photoconductors”) must have the functions of maintaining surface charge in darkness, generating charges upon receiving light, and transporting charges upon receiving light. They are roughly classified into single-layer photoconductors containing a single layer having these functions, and so-called functionally separated multi-layer photoconductors in which a layer contributing mainly to charge generation is laminated on another layer contributing mainly to maintaining surface charges in darkness and to transporting charges upon receiving light.
Electrophotographic image formation by the aforementioned photoconductors is performed through, for example, Carlson's process. This process includes charging a photoconductor in darkness through corona discharging, forming a latent electrostatic image (in response to, for example, characters or pictures of the original document) on the charged photoconductor surface, developing the formed latent electrostatic image with a toner, transferring/fixing the developed toner image onto a medium such as paper. The photoconductor from which the toner image has been transferred is, for example, charge-eliminated, cleaned for removal of the residual toner, and again charge-eliminated with light; and then is subjected to the next cycle.
In recent years, in view of advantages such as flexibility, thermal stability and film formability, organic compounds have been used in electrophotographic photoconductors, and the use of such electrophotographic photoconductors have been put into practice. In particular, functionally separated multi-layer photoconductors have been mainly used which contain, as a photoconductive layer, a charge generation layer containing a charge generation agent and a charge transport layer containing a charge transport agent. Among others, many negatively charged photoconductors have been proposed. The negatively charged photoconductors contain, as a charge generation layer, a layer on which an organic pigment (serving as a charge generation agent) has been deposited or a layer containing the organic pigment dispersed in a resin, and also contain, as a charge transport layer, a layer containing a low-molecular-weight organic compound (serving as a charge transport agent) dispersed in a resin.
Organic compounds have many advantages unlike inorganic compounds. But, at present, none of them satisfactorily meet all the characteristics required for electrophotographic photoconductors. That is, use of organic compounds causes degradation in image quality after repetitive use due to, for example, a decrease in surface potential, an increase in residual potential, and a change in sensitivity potential. The cause of this degradation has not completely been elucidated. One known cause is that a photoconductive layer is adversely affected by oxidative gases (e.g., ozone and NOx) generated from a corona discharger or present in the atmosphere. Specifically, these oxidative gases cause chemical changes in the materials of the photoconductor, or form adsorptive matter on the photoconductive layer surface, thereby changing the characteristics of the photoconductor; for example, causing a drop of resolution due to a decrease in surface potential, an increase in residual potential and a decrease in surface resistance. As a result, image quality is considerably decreased and the service life of the photoconductor is shortened. In one proposed countermeasure against the above-described disadvantages, an antioxidant and a stabilizer are incorporated into the photoconductive layer for preventing such degradation. For example, many patent documents propose the addition of a hindered phenol-based antioxidant or a hindered amine-based antioxidant (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 01-230055). In addition, JP-A Nos. 03-172852, 2002-333731 and 04-56866 propose the addition of an amine derivative. These proposals achieve a certain advantageous effect.
Nevertheless, since the recent copiers and printers have been required for high-speed processing and downsizing, keen demand has arisen for photoconductors with high durability and response. Production of photoconductors having such high response requires a high-molecular-weight charge transport agent or a charge transport agent having a low ionization potential (Ip). However, these charge transport agents have a low resistance to ozone and NOx and thus, the addition of the conventionally used antioxidant is not sufficient.